In future releases of 4th Generation (4G) wireless communications systems and with the introduction of 5th Generation (5G) wireless communications systems, network deployment will gradually change from having been based predominantly on relatively sparsely placed macro and micro cells to a mix of macro cells and densely deployed small cells, so called femto and pico cells. The typical cell radius for each kind of cell is indicated in Table 1.
TABLE 1Cell types and typical cell radiiCell TypeRadiusMacro>2000mMicro200-2000mPico10-200mFemto0-10m
In order for a User Equipment device (UE) in e.g. 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems to be able to operate in a small cell, the UE has to detect the small cell early to prepare the network node (e.g., enhanced or evolved Node B (eNB) or base station) for a potential handover to a new Primary Cell (PCell), Secondary Cell (SCell) configuration (in case of carrier aggregation), and/or Primary Secondary Cell (PSCell) configuration and activation (in case of dual connectivity). The time the small cell can be used by the UE depends for instance on: cell radius, UE speed, and whether range extension by means of enhanced Inter-Cell Interference Coordination (eICIC) or further eICIC (feICIC) is configured by the network node (e.g., the eNB).
Handover to a new PCell, configuration of a new SCell, and configuration and activation of a new PSCell is usually based on measurement reports from the UE, where the UE has been configured by the network node to send measurement reports periodically, at particular events, or a combination thereof. The measurement reports contain physical cell Identity (ID), reference signal strength (Reference Signal Received Power (RSRP)) and reference signal quality (Reference Signal Received Quality (RSRQ)) of the detected cells.
Cell detection by a UE, aiming at detecting and determining cell ID and cell timing of neighbor cells, e.g. in order to find candidates for handover, is facilitated by two signals that are transmitted in each Evolved Universal Terrestrial Radio Access Network (EUTRAN) cell on a 5 millisecond (ms) basis: the Primary and the Secondary Synchronization Signal (PSS and SSS, respectively). Moreover, Reference Signals (RSs) are transmitted in each cell in order to facilitate cell measurements and channel estimation.
In 3GPP LTE systems, the PSS exists in three versions, one for each out of three cell-within-group IDs, and is based on Zadoff-Chu sequences that are mapped onto the central 62 subcarriers and bordered by five unused subcarriers on either side. There are 168 cell groups in total, and information regarding the cell group to which a cell belongs is carried by the SSS, which is based on m-sequences. This signal also carries information on whether the used SSS is transmitted in subframe 0 or subframe 5, which is used for acquiring frame timing. For a particular cell, the SSS is further scrambled with the cell's cell-within-group ID. Hence there are 2×504 versions in total, two for each out of 504 physical layer cell identities. Similar to PSS, SSS is mapped onto the central 62 subcarriers and bordered by five unused subcarriers on either side. The time (subframe)-frequency (subcarrier) grid or layout of synchronization signals in a 3GPP LTE Frequency Division Duplex (FDD) radio frame is shown in FIG. 1. The shown radio frame is wider than the smallest downlink system bandwidth of 1.4 Megahertz (MHz) (72 subcarriers or 6 Resource Blocks (RBs)). Subframes 1-3 and 6-8 may be used for Multi-Broadcast Single Frequency Network (MBSFN) or may be signaled to do so for other purposes, by which a UE cannot expect reference signals in more than the first Orthogonal Frequency Division Multiplexing (OFDM) symbol. The Physical Broadcast Channel (PBCH) (carrying Master Information Block (MIB)) and synchronization signals are transmitted at prior known OFDM symbol positions over the central 72 subcarriers.
Detection of a cell is, as is well-known in the art, based on matched filtering by the UE using the three PSS versions over at least 5 ms of received samples. Correlation peaks in the filter output may reveal synchronization signals from one or more cells. This is referred to as symbol synchronization.
Upon having established symbol synchronization and identified the cell-within-group ID, the next step in cell detection is SSS detection to acquire frame timing and physical layer cell ID. After decoding the SSS, the cell group ID and thereby the full physical layer cell ID is acquired. Moreover, frame timing and cyclic prefix configuration are determined.
The pair of PSS and SSS is always transmitted from the same antenna port at the network node (e.g., eNB), but different pairs may be transmitted from different antenna ports (3GPP Technical Specification (TS) 36.211 V12.3.0, Section 6.11).
Existing methods of cell detection at a UE include:                Non-coherent PSS detection, where matched filtering is carried out individually for each receiver branch, and then the received signal magnitudes (potentially squared to powers) of all receiver branches are added before peak detection is carried out.        Coherent SSS detection, where after having established where the PSS is located in time, the same is used for estimating the radio channel for the cell-to-be-detected before coherently adding the SSS from the different receiver branches and carrying out decoding.        Non-coherent SSS detection, where the timing information from PSS is used but no radio channel is estimated based on it.        
Furthermore, each of these methods may also include interference cancellation of partially or fully overlapping signals from already detected cells (e.g., synchronization and reference signals that are determined from the physical cell ID of a detected cell; decoded and reconstructed PBCH, or other broadcasted channel of a detected cell), whereby the prior known signals are subtracted before carrying out the detection of PSS or decoding of SSS, see for instance commonly held and assigned International Publication No. WO 2014/135204 entitled CHANNEL ESTIMATION FOR INTERFERENCE CANCELLATION.
Network deployments are rapidly moving towards combinations of large and small cells, as illustrated in FIGS. 2A and 2B, where some frequency layers of the cell layout may contain only small cells (e.g., due to physical limitations at high frequencies—e.g., License-Assisted Access (LAA)), and other layers may contain a combination of overlapping large and small cells where the small cells are used to offload the large cells at particular spots (e.g., a Heterogeneous Network (HetNet)). FIGS. 2A and 2B illustrate a deployment scenario example with aggregation using five downlink carriers (F1, . . . , F5). Both HetNet and small cell scenarios introduce challenges for cell detection since the neighbor cells searched for may have very low Signal to Interference and Noise Ratio (SINR).
It is also foreseen that there will be dense deployment of small cells, e.g. in shopping malls, office buildings, airports etc., to provide high capacity to a large number of end users or subscribers (see FIGS. 3A and 3B which are an exemplary sketch of a small cell deployment in such an environment). Here too, cell detection is challenging due to high interference (low SINR).